There has always been a desire to minimize noise around airports where jet aircraft take off. Typically, when a jet aircraft is taking off and climbing in the vicinity of an airport, its engines are set at full or near full throttle and excessive noise becomes a problem. This problem is not so severe, however, in higher altitude flight when the aircraft is in a cruise mode. The problem thus has been to provide a low-noise nozzle for jet propulsion engines which would achieve maximum sound suppression for take-off and climb operations while not introducing excessive performance losses for efficient operation of the aircraft during cruise conditions.
In designing a nozzle which would solve the aforementioned sound problems, designers are continuously confronted with other considerations also associated with aircraft manufacture and operation. Typical of these is the desire to minimize the production and maintenance costs, the weight and the complexity of the nozzle while simultaneously providing the nozzle with additional useful functions, as for example a thrust reversal capability.
There are several prior art patents which utilize panel or flap type elements in forming jet engine nozzles designed to attenuate the noise emitted by a jet pipe of an aircraft engine. Typical of these are U.S. Pat. Nos. 2,978,061 to Keen, 3,153,319 to Young et al., 3,351,155 to Hoch et al., and 3,976,160 to Hoch et al. The '319 patent discloses provision of a number of teeth (4) distributed around the perimeter of a nozzle exit area. The teeth extend in the direction of the issuing jet and have the effect of modifying the boundary of the jet so that the mixing region between jet and atmosphere is altered. Both fixed and movable teeth are disclosed. In such an arrangement, however, the space between teeth acts as a sort of thrust spoiler. Thus, such openings, whether between fixed or movable elements, are clearly undesirable during certain portions of the flight envelope.
The '155 and '160 patents also represent low efficiency nozzles which are undesirable during at least some portions of the flight envelope. In the case of the '155 patent, which discloses a controlled flap-follower flap design, the flaps (1) and follower flaps (2) offer a very irregular shape profile which of course results in a lower efficiency nozzle. On the other hand, the '160 patent uses a square flap type nozzle which represents a heavy design characterized by large boat-tail angles and therefore high boat-tail drag.
Another somewhat `dragging` type design for a nozzle is depicted in the '061 patent. In this design, which consists in one embodiment of a plurality of hollow members (11) forming a plurality of spaces (12) containing two series of six flaps (14), two nozzle settings are disclosed. In the FIG. 1 setting, angularly spaced corrugations provide a silenced nozzle, while in the FIG. 2 setting they form a reduced area frustoconical nozzle of circular outlet contour. This particular nozzle is of a somewhat complex and thus expensive design and includes elements of construction such as hollow members (11) which also result in a somewhat `draggy` design. In addition, the circular outlet contour is of a reduced area, which as will be discussed hereinbelow, is not a preferred arrangement.
An alternative to use of flap type elements can be found in U.S. Pat. No. 3,061,038 to Lawler et al. In this patent, a somewhat heavy, complex and rather inefficient design is disclosed which utilizes a plurality of circumferentially spaced auxiliary nozzle ducts which when extended discharge air rearwardly and outwardly at spaced locations about a nozzle periphery.
Several patents disclose the use of nozzles which provide both noise reduction and thrust reversal capabilities. Exemplary of such patents are U.S. Pat. Nos. 3,133,412 to Westley and 3,973,731 to Thayer. The '731 patent utilizes a flap system arranged in a rectangular exhaust, two of the flaps being adaptable to be placed into a thrust-reversing mode. As noted with respect to the '160 patent, rectangular nozzles and exhaust systems tend to cause undesirable boat-tail drag effects.
The '412 patent on the other hand comprises a segmented frustoconical nozzle as depicted in FIG. 1 thereof wherein each segment is a corrugation in which hinged panels are free to move. Alternate segments have panels (26) which move out between side walls (22). The remaining segments have panels (25) which retract between side walls (21) to provide increased noise suppression without change in nozzle area. FIGS. 1 and 5 show cruise and silencer positions while FIG. 3A shows the nozzle utilized as a thrust reverser. The embodiment depicted in FIG. 5 includes panels (73) and (74) which may be retracted to form a circular nozzle. While the nozzle of the '412 patent provides a means for combining variable noise suppression with a thrust reversing capability, it does not provide a solution to the problem of providing a simple, efficient and economic nozzle whose characteristics include maximum noise suppression in take-off with a maximum of efficiency nozzle at the cruise portions of aircraft flight envelopes, as will be described hereinbelow with reference to the instant invention.
It has long been recognized that the jet from an aircraft engine causes a great deal of noise as it emerges into the atmosphere. If the velocity of the jet is reduced, then the noise created by the engine can be expected to be reduced. However, at take-off, full or nearly full power is required. It has been found, however, that noise suppression can be accomplished at full power through the spreading out of a jet rapidly by increasing the mixing region between the issuing jet and the atmosphere.
One critical aspect of the noise abatement problem is that in the cruise condition, when the air is typically of low density (high altitude), it is most desirable to get maximum thrust by providing a full open and highly efficient nozzle. In such a mode, the need for sound suppression abates and the desire for a maximum efficiency nozzle, i.e., round profile and large area, becomes the desired objective. The problem then is to maximize the take-off (noise abatement) efficiency and cruise efficiency of the nozzle, while minimizing cost, complexity, drag, etc. and providing a thrust reversal option.
In view of the shortcomings of the aforementioned prior art devices, there is a need for an improved jet exhaust nozzle for maximizing take-off sound suppression and cruise efficiency.
It is a primary object then of the instant invention to provide a low-noise nozzle for aircraft jet propulsion engines which is adaptable to achieve both maximum noise suppression during take-off and climb portions of the flight envelope and minimum performance losses during cruise conditions.
It is an additional object of the present invention to provide a low-noise nozzle for jet propulsion engines that is economic to produce and maintain, lightweight and simple.
It is yet another object of the present invention to provide a low-noise nozzle for jet propulsion engines that performs added functions other than noise reduction, as for example providing a thrust reversal capability.